Preheat oven to 180°C/350°F. Lightly butter a 20cm (8in) baking pan and line it with aluminum foil leaving an overhang on two opposite sides. Butter the foil.

Start with the crust: in a medium bowl, combine melted butter with sugar, vanilla and salt. Add flour and mix until just incorporated. Press dough evenly over bottom of pan. Bake 25 to 30 minutes, or until crust is fully baked, well-browned at the edges and golden brown in the center.
Now, the topping: while crust is baking, stir together sugar and flour in a large bowl until well-mixed. Whisk in eggs. Stir in zest and juices. When crust is ready, reduce heat to 150°C/300°F, slide rack with pan out and pour filling onto hot crust. Bake 20 to 25 minutes longer, or until topping no longer jiggles when pan is tapped.

Remove from oven to a wire rack to cool completely. Lift up foil liner and transfer bars to a cutting board. If surface is covered with a thin layer of moist foam (not unusual), blot surface gently with a paper towel to absorb excess moisture; repeat with a fresh piece of paper towel if necessary.
Using a long, sharp knife, cut bars into 16 or 24 daintier bars and sift powdered sugar over bars, if desired. Stored in an airtight container, bars can be refrigerated for up to 3 days.